Since the 1950's, the dental industry has invested a great deal of effort into the research and development of tooth-colored alternatives to metal amalgam for the restoration of teeth. The use of acrylic (e.g., poly(methyl methacrylate) or PMMA) was a first step toward the use of polymer technology for tooth restoration. Many of the current dental restorative systems are based on dimethacrylate monomer resins containing silane-treated inorganic filler particles (such as barium, strontium, zirconium glasses or quartz) and are cured via free-radical polymerization.
Methacrylate composites were first introduced as two-component systems that were chemically cured. One component of the system typically contains a peroxide, the other an amine. When mixed together the two initiator components react to create free radicals and initiate polymerization of the methacrylate matrix. This procedure requires substantial mixing time before application and offers limited contouring time before the composite is cured. The introduction of initiator systems that produced free radicals via visible light (400-1000 nm) absorption attempted to address these problems by permitting the use of single-component restorative systems that were cured after contouring.
Unfortunately these light-cured methacrylate restoratives can exhibit significant shrinkage during photopolymerization, which can lead to the build-up of stress within the composite and at the composite-tooth interface. These stresses can become high enough to result in cusp fracture, marginal failure, and/or post-operative sensitivity. For this reason, incremental placement and curing of light curing composites is a common dental practice. This process allows for minimization of stress/shrinkage related complications, but also increases the amount of working time required for a successful restoration.
Previous research has focused on the development of low-shrink restoratives as an alternative to incremental placement techniques. The use of aliphatic epoxy monomers as dental resins is one promising solution. These materials generally have, on the average, at least 1 polymerizable epoxy group per molecule, usually at least 2, and sometimes as many as 4 or more polymerizable epoxy groups per monomer. These epoxies utilize a cationic ring-opening polymerization curing mechanism.
Epoxy-containing compounds are known to be curable using various cationic initiator systems. For example, ternary photoinitiator systems comprising an iodonium salt, a visible light sensitizer, and an electron donor have been developed for curing of epoxy resins and epoxy/polyol resins. Although these systems have shown much promise, it is desirable to increase cure speed and depth of cure, and to provide for better color formation and sensitivity to temperature. Thus, a need remains for photopolymerizable compositions capable of providing satisfactory cure speed and depth of cure, while at the same time minimizing unwanted color formation and exhibiting good color stability.